/* Copyright (c) 2012 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package it.localhost.trafficdroid.common.billing;

import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

import android.text.TextUtils;
import android.util.Base64;
import android.util.Log;

/**
 * Security-related methods. For a secure implementation, all of this code
 * should be implemented on a server that communicates with the application on
 * the device. For the sake of simplicity and clarity of this example, this code
 * is included here and is executed on the device. If you must verify the
 * purchases on the phone, you should obfuscate this code to make it harder for
 * an attacker to replace the code with stubs that treat all purchases as
 * verified.
 */
public class Security {
	private static final String TAG = "IABUtil/Security";
	private static final String KEY_FACTORY_ALGORITHM = "RSA";
	private static final String SIGNATURE_ALGORITHM = "SHA1withRSA";

	/**
	 * Verifies that the data was signed with the given signature, and returns
	 * the verified purchase. The data is in JSON format and signed with a
	 * private key. The data also contains the {@link PurchaseState} and product
	 * ID of the purchase.
	 * 
	 * @param base64PublicKey
	 *            the base64-encoded public key to use for verifying.
	 * @param signedData
	 *            the signed JSON string (signed, not encrypted)
	 * @param signature
	 *            the signature for the data, signed with the private key
	 */
	public static boolean verifyPurchase(String base64PublicKey, String signedData, String signature) {
		if (signedData == null) {
			Log.e(TAG, "data is null");
			return false;
		}
		boolean verified = false;
		if (!TextUtils.isEmpty(signature)) {
			PublicKey key = Security.generatePublicKey(base64PublicKey);
			verified = Security.verify(key, signedData, signature);
			if (!verified) {
				Log.w(TAG, "signature does not match data.");
				return false;
			}
		}
		return true;
	}

	/**
	 * Generates a PublicKey instance from a string containing the
	 * Base64-encoded public key.
	 * 
	 * @param encodedPublicKey
	 *            Base64-encoded public key
	 * @throws IllegalArgumentException
	 *             if encodedPublicKey is invalid
	 */
	public static PublicKey generatePublicKey(String encodedPublicKey) {
		try {
			byte[] decodedKey = Base64.decode(encodedPublicKey, Base64.DEFAULT);
			KeyFactory keyFactory = KeyFactory.getInstance(KEY_FACTORY_ALGORITHM);
			return keyFactory.generatePublic(new X509EncodedKeySpec(decodedKey));
		} catch (NoSuchAlgorithmException e) {
			throw new RuntimeException(e);
		} catch (InvalidKeySpecException e) {
			Log.e(TAG, "Invalid key specification.");
			throw new IllegalArgumentException(e);
		}
	}

	/**
	 * Verifies that the signature from the server matches the computed
	 * signature on the data. Returns true if the data is correctly signed.
	 * 
	 * @param publicKey
	 *            public key associated with the developer account
	 * @param signedData
	 *            signed data from server
	 * @param signature
	 *            server signature
	 * @return true if the data and signature match
	 */
	public static boolean verify(PublicKey publicKey, String signedData, String signature) {
		Signature sig;
		try {
			sig = Signature.getInstance(SIGNATURE_ALGORITHM);
			sig.initVerify(publicKey);
			sig.update(signedData.getBytes());
			if (!sig.verify(Base64.decode(signature, Base64.DEFAULT))) {
				Log.e(TAG, "Signature verification failed.");
				return false;
			}
			return true;
		} catch (NoSuchAlgorithmException e) {
			Log.e(TAG, "NoSuchAlgorithmException.");
		} catch (InvalidKeyException e) {
			Log.e(TAG, "Invalid key specification.");
		} catch (SignatureException e) {
			Log.e(TAG, "Signature exception.");
		}
		return false;
	}
}
